The U.S. aluminum industry is one of the largest in the world with about 2.5 million metric tons of primary aluminum produced in 2005. Presently, the aluminum industry relies on three major processes for primary aluminum production: alumina refining from bauxite, anode production, and aluminum smelting by electrolysis in the Hall-Heroult process. The Hall-Heroult electrolytic cells electrochemically reduce alumina to aluminum metal via carbon anodes and molten aluminum cathodes in the smelting process. Smelting is the most energy intensive step in primary aluminum production which accounts for between 2% and 3% of the electricity used in the U.S. every year (about 15 kWh/kg aluminum produced). Smelting also results in a variety of emissions, effluents, by-products and solid wastes. Greenhouse gases are a major pollutant from aluminum production and are caused by fossil fuel consumption, carbon anode consumption, and perfluorocarbons from anode effects. Emissions from anode production include particulates, fluorides, polycyclic aromatic hydrocarbons (PAH) and sulfur dioxide (SO2). Emissions from aluminum smelting include carbon monoxide (CO), carbon dioxide (CO2), SO2, fluorides, perfluorocarbons (PFCs, e.g., CF4, C2F6), and PAH. It would be advantageous to lower costs and reduce waste to remain competitive with foreign producers. The smelting step is a priority area for improvement because of high energy use and undesirable emissions and by-products implicated in climate change.
Carbothermic reduction of aluminum is an alternative process for aluminum production using a chemical reduction reaction in a reactor. Carbothermic processes require much less physical space than the Hall-Heroult electrolytic reduction process. The carbothermic reduction process of aluminum production, as a replacement for the Hall-Heroult process, would result in decreased electrical consumption. Long term estimates suggest the carbothermic process could reduce energy requirement by over 30% to about 8.5 kWh/kg. The carbothermic aluminum production process would also eliminate perfluorocarbon emissions resulting from carbon anode effects, hazardous spent potliners, and hydrocarbon emissions associated with baking of consumable carbon anodes. This alternative carbothermic aluminum production process would be more energy efficient and have less environmental impact than traditional aluminum production plants.
The direct carbothermic reduction of alumina has been described in U.S. Pat. No. 2,974,032 (Grunert et al.) and U.S. Pat. No. 6,440,193 B1 (Johansen et al.) as well as in Proceedings 6th Conference on Molten Slags, Fluxes and Salts, Edited by S. Seetharaman and D. Sichen “Carbothermic Aluminum”, K. Johansen, J. Aune, M. Bruno and A. Schei, Stockholm, Sweden-Helsinki Finland, Jun. 12-17, 2002.
It has long been recognized that the overall aluminum carbothermic reduction reaction:Al2O3+3C→2Al+3CO  (1)
takes place, or can be made to take place, generally in steps such as:2Al2O3+9C→Al4C3+6CO (vapor)  (2)Al4C3+Al2O3→6Al+3CO (vapor)  (3)Al2O3+2C→Al2O (vapor)+2CO (vapor)  (4)Al2O3+4Al→3Al2O (vapor)  (5), andAl→Al (vapor)  (6).
Reaction (2), the slag producing step, generally takes place at temperatures between 1900° C. and 2000° C. Reaction (3), the aluminum producing step, generally takes place at temperatures above about 2050° C. and requires substantial heat input. A large quantity of aluminum vapor species are formed during reactions (2) and (3). In addition to the species shown in reactions (2) and (3), volatile species including gaseous Al, reaction (6), and gaseous aluminum suboxide (Al2O) are formed in reaction (4) or (5). In the overall carbothermic reduction process, the Al2O and Al gases can be recovered by reacting them with carbon in a separate reactor usually called the vapor recovery unit or vapor recovery reactor (VRR).
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. No. 4,099,959 (Dewing et al.), U.S. Pat. Nos. 4,033,757 and 4,388,107 (both Kibby). U.S. Pat. Nos. 4,334,917 and 4,533,386 (both Kibby), U.S. Pat. No. 6,440,193 (Johansen and Aune), and U.S. Patent Publication No. US2006/0042413 (Fruehan).
One prior method of continuous carbothermic aluminum production method teaches reaction of C and Al2O3 in a first stage compartment to produce a slag containing Al2O3+Al4C3, which underflows a baffle into a second stage compartment where the Al4C3 is reduced to Al through increased temperature. A disadvantage to this process is that the aluminum metal produced contains a high concentration of C, up to saturation with aluminum carbide (˜8% C) and does not readily flow out the reactor. Another disadvantage of prior staged continuous carbothermic process is that the method requires movement of the product out of the reactor to an external decarbonization unit. Another disadvantage is that extensive back-mixing may occur between the slag making and metal making portions of the reactor. Furthermore, there is a deficiency of aluminum carbide during metal making.
FIG. 1 is a simplified illustration of another prior art carbothermic reaction process that produces Al, recovers Al, Al2O and CO in the off-gases as Al4C3, Al2O3 and slag, and passes this material to the smelting furnace. In FIG. 1, gas flows are shown as dashed lines and flows of solids and molten substances are shown as solid lines. In FIG. 1, the off-gases 3, 4 from carbothermic smelting furnace are recovered during a first stage 1 and second stage 2 and forwarded to an enclosed off-gas reactor 5. The Al-components of the off-gas entering the reactor 5 react with carbon 7 to form one or more of Al4C3, Al2O3 and Al4C3—Al2O3 slag Al4C3 is fed to the furnace during the second stage 2, the metal making step.
One disadvantage of this method is that, although Al4C3 is needed in stage 2 (the metal making step) to partially satisfy the chemistry of the process, the inflow of Al4C3 from the vapor recovery reactor may also add other products to the reactor during the metal making step (e.g., unreacted carbon, slag and Al2O3). The addition of these other solid vapor recovery reactor materials during the metal making step is undesirable for several reasons, including that the addition of unreacted carbon will increase the amount of generated carbon monoxide (CO), causing excess aluminum suboxide and aluminum vaporization. There is also a limited tolerance for efficient recycling of variable vapor recovery solid discharge.